Aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid are used to produce a variety of polyester products, important examples of which are poly(ethylene terephthalate) and its copolymers. These aromatic dicarboxylic acids may be synthesized by the catalytic oxidation of the corresponding dialkyl aromatic compound. For example, terephthalic acid (TPA) and isophthalic acid (IPA) may be produced by the liquid phase oxidation of p-xylene and m-xylene, respectively.
These processes typically comprise feeding one or more dialkyl aromatic hydrocarbons, fresh and/or recycled solvent or reaction medium, and catalyst components to a reactor to which a molecular oxygen-containing gas also is fed, typically near the bottom of the reactor. Conventional liquid-phase oxidation reactors are equipped with agitation means for mixing the multi-phase reaction medium. Agitation of the reaction medium is supplied in an effort to promote dissolution of molecular oxygen into the liquid phase of the reaction medium, and to facilitate contact between the dissolved oxygen and the dialkyl aromatic hydrocarbon in the reaction medium. Agitation of the reaction medium undergoing liquid-phase oxidation is frequently provided by mechanical agitation means in vessels such as, for example, continuous stirred tank reactors (CSTRs). Bubble column reactors provide an attractive alternative to CSTRs and other mechanically agitated oxidation reactors.
In these processes, bubble column reactors may be used having relatively high height to diameter ratios. The oxygen-containing process gas rising through the liquid contents of the reactor results in agitation of the reaction mixture. Alternatively, continuous stirred tank reactors may be used, typically having a lower height to diameter ratio than bubble column reactors. The aromatic dicarboxylic acid produced may be removed continuously through an exit port as a slurry. Process gas containing excess oxygen, along with solvent decomposition products, may be removed through an upper exit port typically located at or near the top of the reactor. The heat of reaction may also be removed through the upper exit port by vaporization of the process solvent and water generated by the reaction.
Thus, in one example of such a process, p-xylene is oxidized to produce terephthalic acid. The p-xylene may be continuously or batchwise oxidized in the primary oxidation reactor in the liquid phase, in the presence of an oxygen-containing gas such as air. In such a process, p-xylene, an oxidation catalyst composition, a molecular source of oxygen, and a solvent such as aqueous acetic acid are combined as a reaction medium in the reactor to produce a crude terephthalic acid (CTA) reaction product. Typical oxidation catalyst compositions include a cobalt compound and a manganese compound, usually in combination with a promoter such as a bromine compound. See, for example, U.S. Pat. Nos. 2,833,816, 3,089,906, and 4,314,073, the disclosures of which are incorporated herein by reference. The process conditions are highly corrosive due to the acetic acid and bromine, and titanium is typically used in the process equipment. See, for example, U.S. Pat. No. 3,012,038, incorporated herein by reference. Acetaldehyde may be used as a promoter in place of bromine, in which case titanium materials need not be used. Acetaldehyde is also useful as an initiator. Because the liquid-phase oxidations of dialkyl aromatic compounds just described are highly exothermic reactions, they are commonly carried out in vented reaction vessels, the heat of reaction being removed by vaporization of the process solvent through the upper exit port.
The resulting CTA is not very soluble in the acetic acid solvent under the reaction conditions, and precipitates from the solvent to form a suspension. This crude terephthalic acid suspension includes terephthalic acid solids, a solvent acting as the suspending medium for the solids and containing a small amount of dissolved terephthalic acid; catalyst components; unreacted p-xylene; incompletely oxidized intermediate oxidation products such as para-tolualdehyde (p-TA1), para-toluic acid (p-TA), and 4-carboxybenzaldehyde (4-CBA); and organic impurities such as fluorenones that are known to cause discoloration. The crude terephthalic acid composition is discharged from the oxidation zone and subjected to any of several mother liquor exchange, separation, purification, or recovery methods, with the recovered solvent and catalyst composition being recycled directly back to the oxidation reaction or after processing, such as by catalyst recovery or solvent purification. It is desirable to minimize the amount of incompletely oxidized intermediates and the colored impurities, to reduce the subsequent purification requirements.
Other by-products of the liquid phase oxidation which are partially or completely removed from the reaction mixture in the oxidation reactor are the off-gases, which include water, solvent, unreacted oxygen and other unreacted gases found in the source of the molecular oxygen gas such as nitrogen and carbon dioxide, and additional amounts of carbon dioxide and carbon monoxide that are oxidative losses resulting in part from the catalytic decomposition of the solvent and other oxidizable compounds under the oxidation conditions. The off-gases are vented at the overhead of the oxidation reactor to a distillation column or a condenser to separate the solvent from the other off-gases such as water, carbon dioxide, carbon monoxide, nitrogen, gaseous bromine compounds such as methyl bromide, etc.
Although it is desirable to recover and recycle as much solvent as possible, the solvent is oxidatively decomposed to some extent into its constituent gaseous products, carbon dioxide and carbon monoxide, requiring a fresh source of make-up solvent. This oxidative decomposition is often referred to in the industry as solvent burn or acid burn, and is generally believed to be responsible in part for the formation of carbon oxides, although a portion of the carbon oxides produced is also the result of oxidative decomposition of the dialkyl aromatics or intermediate reaction products. Controlling or reducing formation of carbon oxides would significantly lower the operating costs of the oxidation process, by allowing a greater amount of solvent to be recovered and recycled back to the oxidation zone, and possibly also by reducing yield loss from the oxidative decomposition of the aromatic reactants. However, a reduction in carbon oxides formation should not come at the expense of significantly reduced yield or conversion, or an increase in the amount of incomplete oxidation products in the crude mixture, and if possible, it would be desirable to simultaneously reduce carbon oxides formation and increase the conversion. Typically, however, increased conversion is accompanied by an increase in carbon oxides formation.
U.S. Pat. No. 3,920,735 discloses a method of oxidizing di- or trimethylbenzenes with molecular oxygen to form benzene di- or tri-carboxylic acids under liquid phase conditions using catalyst systems that include cobalt, bromine, and zirconium; or cobalt, manganese, bromine, and zirconium. According to the disclosure, these catalyst systems must contain at least 20 percent manganese.
G. S. Bezhanishvili and V. A. Nezdominov, Neftepererabotka I Neftekhimiya (Moscow, Russian Federation) 1983, 4, 39-40, studied the effects of the addition of manganese and zirconium on a cobalt-bromide catalyst used for the oxidation of para-xylene at atmospheric pressure and temperatures less than 100° C.
U.S. Pat. No. 4,992,580 discloses that the addition of molybdenum to an oxidation catalyst system that includes cobalt increases the catalytic activity of the catalyst system. The molybdenum is said to activate the cobalt moiety.
U.S. Pat. No. 5,359,133 discloses a multi-stage process for producing benzenedicarboxylic acid isomers that includes an oxidation step wherein xylene isomer is partially oxidized with molecular oxygen or molecular oxygen-containing gas in the presence of a catalyst system composed of cobalt, manganese, bromine, and at least one selected from nickel, chromium, zirconium and cerium in lower aliphatic carboxylic acid.
PCT Publn. No. WO 00/37406 discloses the liquid phase oxidations of alkyl aromatic hydrocarbons, using oxygen-enriched gas, in the presence of a catalyst of cobalt, manganese, and bromide, and one or more than one type of transition metal or lanthanide metal component.
PCT Publn. No. WO 00/66529 discloses that the use of a feed gas containing both oxygen and carbon dioxide for the oxidation of alkyl aromatic substrates or their partially oxidized intermediates to produce carboxylic acid products improves the yield and quality of the resulting product. The carbon dioxide is said to function as a co-oxidant along with oxygen. The document suggests that in addition to a catalyst system comprising cobalt, manganese, and bromine, an additional transition metal or lanthanide series metal may be introduced when deemed necessary.
There remains a need in the art for aromatic oxidation processes that achieve improved conversion, while minimizing carbon oxides formation. These and additional advantages are obtained by the present invention, as further described below.